1. Field of the Invention
This invention relates to the fabrication of a perpendicular magnetic recording (PMR) write head whose main pole is surrounded on all sides by shields formed of magnetic material. In particular it relates to the formation of such shields using layers of the same magnetic material so that a consistent fabrication process can be employed and so that a corresponding consistent performance can be obtained.
2. Description of the Related Art
The increasing need for high recording area densities (up to 500 Gb/in2) is making the perpendicular magnetic recording head (PMR head) a replacement of choice for the longitudinal magnetic recording head (LMR head).
By means of fringing magnetic fields that extend between two emerging pole pieces, longitudinal recording heads form small magnetic domains within the surface plane of the magnetic medium (hard disk). As recorded area densities increase, these domains must correspondingly decrease in size, eventually permitting destabilizing thermal effects to become stronger than the magnetic interactions that tend to stabilize the domain formations. This occurrence is the so-called superparamagnetic limit. Recording media that accept perpendicular magnetic recording, allow domain structures to be formed within a magnetic layer, perpendicular to the disk surface, while a soft magnetic underlayer (SUL) formed beneath the magnetic layer acts as a stabilizing influence on these perpendicular domain structures. Thus, a magnetic recording head that produces a field capable of forming domains perpendicular to a disk surface, when used in conjunction with such perpendicular recording media, is able to produce a stable recording with a much higher area density than is possible using standard longitudinal recording.
Since their first use, the PMR head has evolved through several generations. Initially, the PMR head was a monopole, but that design was replaced by a shielded head design with a trailing edge shield (TS), which provides a high field gradient in the down-track direction to facilitate recording at high linear densities. Side shields (SS) then began to be used in conjunction with the trailing edge shields, because it was necessary to eliminate the fringing side fields in order to increase writing density still further. To further reduce the fringing in the down-track direction, thus reducing the length of the “write bubble” (the iso-field contour) down the track and improving write performance at a skew angle, a leading edge shield (LS) was also proposed, making the write head four-side shielded.
Despite the aforementioned advantages for the four-sided shielded design, it does require additional design optimizations for all the shield layers. It is believed that a high saturation magnetic moment (Bs) seed layer, such as CoFe with a Bs of 2.4 T (Tesla), for the TS would improve the down-track field gradient. It is also traditionally believed that the LS and TS are somewhat “non-critical” layers and they are often formed of very low moment material such as permalloy. As a result, there will be a significant mismatch in material compositions and moments for these layers, all of which are exposed at the ABS (air bearing surface) of the write head.
Several issues may arise as a result of materials and moment mismatches. First, the pole tip recession/protrusion may be very different between the layers, as a result of hardness differences between the materials and lapping rate variations during the slider lapping process that defines the final ABS. This may affect the magnetic spacing between the write pole and media during write operation, thereby adversely affecting performance. For example, AFM (atomic force microscopy) images show higher protrusion of the TS/SS seed layer relative to the surrounding materials. The seed layer has a Bs=2.2 T, whereas for the TS/SS shield materials themselves Bs=1.9 T. Another downside of higher seed layer protrusion could be erasures from the shield corners due to closeness of the seed layers to the media.
Another issue associated with the material/moment mismatches between different shield layers is the formation of domain walls at the layer interfaces that may cause wide area track erasures (WATE). This could be a result of different material magnetostrictions causing different domain configurations in neighboring layers, which, in turn create domain walls at the interfaces, or it could just be due to the moment mismatches producing magnetic charges at the interfaces which produce stray fields.
Magnetic force microscopy applied to shield configurations with WS1 (trailing shield) and PP3 (plated top layer) layers formed of materials having Bs=1.8 T and 1.0 T show evidence of domain walls propagating from the MP region upward and stopping at the interface between the materials. On the other hand, wrap-around shield configurations with all shields, SS, WS1 and PP3 made of the same Bs material, show no such domain walls on the ABS and there is no WATE.
An additional disadvantage of using low Bs materials in the LS and pole yoke layers is that in order to conduct the same amount of magnetic flux as a material with twice the value of Bs, would require twice the thickness. For example, the use of low Bs Ni80Fe20 vs. a NiFe, CoFe or CoNiFe alloy with a Bs of about 2.0 T. Larger metal volumes required of the lower Bs metals will cause larger protrusions during temperature increases either due to ambient increases or the heat generated by energizing currents.
Issues relevant to shield materials are described in the prior arts. For example, Terris et al. (U.S. Pat. No. 7,068,453) discloses side and trailing shields formed of a soft magnetic material.
Gao et al. (U.S. Pat. No. 7,441,325) discloses a trailing shield formed of NiFe.
Nix et al. (U.S. Pat. No. 7,367,112) teaches the formation of a main pole with trailing and side shields.
Guan et al. (U.S. Pat. No. 7,322,095, assigned to the present assignee) teaches a wrap-around shield, as do Jiang et al. (US Patent Application 2009/0154026) and Hsiao et al. (US Patent Application 2009/0154019).
None of the prior art cited above address the problem addressed by the present invention nor do they disclose the structures and materials of the present invention.